Method for producing liposomes with increased percent of compound encapsulated

ABSTRACT

The efficiency of encapsulating a drug into a liposomal formulation is increased by use of a lipid having a carbon chain containing from about 13 to about 28 carbons during preparation of the liposomes. Preferably the liposomes are multivesicular liposomes.

BACKGROUND OF THE INVENTION

This invention relates to liposomal formulations of compounds such asdrugs. More particularly this invention relates to methods of increasingthe encapsulation of desired compounds in liposomal formulations and themethods of making them.

When phospholipids and many other amphipathic lipids are dispersedgently in an aqueous medium they swell, hydrate and spontaneously formmultilamellar concentric bilayer vesicles with layers of aqueous mediaseparating the lipid bilayers. These systems commonly are referred to asmultilamellar liposomes or multilamellar vesicles (MLV) and usually havediameters of from 0.2 μm to 5 μm. Sonication of MLV results in theformation of small unilamellar vesicles (SUV) with diameters usually inthe range of 20 to 100 nm, containing an aqueous solution in the core.Multivesicular liposomes (MVL) differ from multilamellar liposomes inthe random, non-concentric arrangement of chambers within the liposome.Amphipathic lipids can form a variety of structures other than liposomeswhen dispersed in water, depending on the molar ratio of lipid to water,but at low ratios the liposome is the preferred structure.

The physical characteristics of liposomes generally depend on pH andionic strength. They characteristically show low permeability to ionicand polar substances, but at certain temperatures can undergo agel-liquid crystalline phase (or main phase) transition dependent uponthe physical properties of the lipids used in their manufacture whichmarkedly alters their permeability. The phase transition involves achange from a closely packed, ordered structure, known as the gel state,to a loosely packed, less-ordered structure, known as the liquidcrystalline state.

Various types of lipids differing in chain length, saturation, and headgroup have been used in liposomal drug formulations for years, includingthe unilamellar, multilamellar, and multivesicular liposomes mentionedabove. One of the major goals of the field is to develop liposomalformulations for sustained release of drugs and other compounds ofinterest, and liposomal formulations from which the rate of release ofthe encapsulated compound can be controlled.

These goals are important and many studies have been undertaken towardsachieving them. Another less recognized goal, increasing the yield ofproduct from a liposomal formulation used as a delivery agent, has verypractical benefits as well, particularly to the pharmaceutical industry.For instance, increasing the percent of drug encapsulated in liposomalformulations can result in increased yield and substantial cost savings.In the case of liposomal drug formulations, it is also desirable to havethe highest possible percent of drug encapsulated for any givenlipid:drug ratio to avoid the need for injecting highly viscousformulations or large volumes into the patient in order to achieve adesired dosage. If a process results in a high percentage of compoundencapsulated but yields a product with a low drug:lipid ratio, it isgenerally necessary that the formulation have a high lipocrit (analogousto hematocrit) in order to satisfy a specified drug dose, or provide atherapeutically effective amount of a biologically active substance viaan injection. Analogous to hematocrit, lipocrit is a measure of thepercent volume occupied by the liposomes relative to the total volume ofthe liposome suspension. Yet, such formulations are difficult toadminister by injection because of their high viscosities. Thus, theneed exists for more and better methods for obtaining liposomalformulations that maximize the efficiency of a drug encapsulation toachieve a low lipid:drug ratio.

SUMMARY OF THE INVENTION

A method is provided for increasing the percent of a compound, such as adrug, that is encapsulated in a liposomal formulation comprisingincreasing the number of carbons in the carbon chain of at least onelipid in the lipid component of the liposome, wherein the chemicalstructures of the two lipids are otherwise substantially similar. Thepreferred length of carbon chain in at least one lipid in amultivesicular liposome is increased to an integer in the range fromabout 13 to about 28, most preferably about 13 to 22. The preferredlipids of increased carbon chain length are phospholipids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the drug release profile at 4° C. ofmultivesicular liposomes made with DC18:1PC. The encapsulated drug iscytarabine.

FIG. 2 is a graph illustrating the drug release profile at 4° C. ofmultivesicular liposomes made with DC14:0PC. The encapsulated drug iscytarabine.

FIG. 3 is a graph illustrating the drug release profile at 4° C. ofmultivesicular liposomes made with DC16:0PC. The encapsulated drug iscytarabine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method for increasing the efficiencywith which a compound is encapsulated into a liposomal formulation. Ithas surprisingly been discovered that, in liposomal formulations, theencapsulation efficiency of the active substance can be significantlyincreased by increasing the number of carbons in the carbon chain of atleast one of the amphipathic lipids used in preparation of the liposomalformulation. This invention is particularly useful in the pharmaceuticalindustry for increasing the efficiency with which a pharmacologicallyactive amount of a biologically active compound is encapsulated in aliposome without substantially increasing the lipocrit of theformulation for a given drug dose. The method of the invention is usefulfor increasing the encapsulation efficiency of any type of liposome, butin a preferred embodiment a method for increasing encapsulationefficiency during manufacture of a multivesicular liposomal formulationis provided.

There are at least three types of liposomes. The term "multivesicularliposomes (MVL)" as used throughout the specification and claims meansman-made, microscopic lipid vesicles comprising lipid membranesenclosing multiple non-concentric aqueous chambers. In contrast,"multilamellar liposomes or vesicles(MLV)" have multiple "onion-skin"concentric membranes, in between which are shell-like concentric aqueouscompartments. Multilamellar liposomes and multivesicular liposomescharacteristically have mean diameters in the micrometer range, usuallyfrom 0.5 to 25 μm. The term "unilamellar liposomes or vesicles (ULV)" asused herein refers to liposomal structures having a single aqueouschamber, usually with a mean diameter range from about 20 to 500 nm.

Multilamellar and unilamellar liposomes can be made by severalrelatively simple methods. The prior art describes a number oftechniques for producing ULV and MLV (for example U.S. Pat. Nos.4,522,803 to Lenk; 4,310,506 to Baldeschweiler; 4,235,871 toPapahadjopoulos; 4,224,179 to Schneider, 4,078,052 to Papahadjopoulos;4,394,372 to Taylor 4,308,166 to Marchetti; 4,485,054 to Mezei; and4,508,703 to Redziniak).

By contrast, production of multivesicular liposomes requires severalprocess steps. Briefly, the preferred method for making MVL is asfollows: The first step is making a "water-in-oil" emulsion bydissolving at least one amphipathic lipid and at least one neutral lipidin one or more volatile organic solvents for the lipid component, addingto the lipid component an immiscible first aqueous component and abiologically active substance to be encapsulated, and optionally adding,to either or both the lipid component and the first aqueous component,an acid or other excipient for modulating the release rate of theencapsulated biologically active substances from the MVL. The mixture isemulsified, and then mixed with a second-immiscible aqueous component toform a second emulsion. The second emulsion is mixed eithermechanically, by ultrasonic energy, nozzle atomization, and the like, orby combinations thereof, to form solvent spherules suspended in thesecond aqueous component. The solvent spherules contain multiple aqueousdroplets with the substance to be encapsulated dissolved in them (seeKim et al., Biochem. Biophys. Acta, 728:339-348, 1983). For acomprehensive review of various methods of ULV and MLV preparation,refer to Szoka, et al. Ann. Rev. Biophys. Bioeng. 9:465-508, 1980.

The term "solvent spherule" as used throughout the specification andclaims means a microscopic spheroid droplet of organic solvent, withinwhich are multiple smaller droplets of aqueous solution. The solventspherules are suspended and totally immersed in a second aqueoussolution.

The term "neutral lipid" means an oil or fat that has nomembrane-forming capability by itself and lacks a hydrophilic "head"group.

The term "amphipathic lipid" means a molecule that has a hydrophilic"head" group and hydrophobic "tail" group and has membrane-formingcapability.

The term "zwitterionic lipid" means an amphipathic lipid with a netcharge of zero at pH 7.4.

The term "anionic lipid" means an amphipathic lipid with a net negativecharge at pH 7.4.

The term "cationic lipid" means an amphipathic lipid with a net positivecharge at pH 7.4.

For making multivesicular liposomes, it is required that at least oneamphipathic lipid and one neutral lipid be included in the lipidcomponent. The amphipatnic lipids can be zwitterionic, anionic, orcationic lipids. Examples of zwitterionic amphipathic lipids arephosphatidylcholines, phosphatidylethanolamines, sphingomyelins etc.Examples of anionic amphipathic lipids are phosphatidylglycerols,phosphatidylserines, phosphatidylinositols, phosphatidic acids, etc.Examples of cationic amphipathic lipids are diacyltrimethylammoniumpropane and ethyl phosphatidylcholine. Examples ofneutral lipids include diglycerides, such as diolein, dipalmitolein, andmixed caprylin-caprin diglycerides; triglycerides, such as triolein,tripalmitolein, trilinolein, tricaprylin, and trilaurin; vegetable oils,such as soybean oil; animal fats, such as lard and beef fat; squalene;tocopherol; and combinations thereof. Additionally, cholesterol or plantsterols can be used in making multivesicular liposomes.

As used herein, "percent encapsulation of drug, or other compound" meansthe ratio of the amount of compound to be encapsulated in the finalsuspension of the liposome manufacturing process to the total amount ofcompound to be encapsulated used in the first aqueous solution of theprocess multiplied by 100. ##EQU1##

As used herein, "lipocrit," which is defined in analogy to hematocrit,means the ratio of the volume occupied by the liposomes to the totalsuspension volume multiplied by 100. ##EQU2##

As used herein, "percent free drug" means the ratio of the amount ofdrug exterior to the liposomes in the final liposome suspension to thetotal amount of drug in the final suspension (the final product)multiplied by 100. ##EQU3##

The methods for determining these parameters are illustrated in Example2 of this application.

As used herein the term "therapeutically effective amount" means theamount of a biologically active substance necessary to induce a desiredpharmacological effect. The amount can vary greatly according to theeffectiveness of a particular active substance, the age, weight, andresponse of the individual host as well as the nature and severity ofthe host's symptoms. Accordingly, there is no upper or lower criticallimitation upon the amount of the active substance. The therapeuticallyeffective amount to be employed in the present invention can readily bedetermined by those skilled in the art.

In the method of the present invention, the encapsulation efficiency ofany given liposomal formulation employing short chain amphipathiclipids, having 12 or less carbons in the carbon chain, can be increasedby increasing the chain length within any given lipid used in theformulation of the liposome, generally from a carbon chain length of 13to about 28 carbons, and preferably from about 18 to about 22 carbons.This general rule holds whether the carbon chain of the amphipathiclipid to be changed is saturated, or whether it contains one or moredouble bonds. Generally, however, in selecting the lipids to be used informulating a multivesicular liposome it should be kept in mind that itis possible to use an organic solvent with a lower boiling point whenutilizing a lipid with a given number of carbons in the carbon chain, ifthe lipid contains at least one double bond in the carbon chain. Thepreferred amphipathic lipids for use in making the multivesicularliposomes with increased encapsulation efficiency are phospholipidsbecause phospholipids are natural lipids found in the body.

A representative list of long chain amphipathic lipids preferred for usein the practice of this invention follows. This list is illustrative andnot intended to in any way limit the scope of the invention. Alsoincluded are the abbreviations used to refer to the phospholipids inthis application.

DOPC or DC18:1PC=1,2-dioleoyl-sn-glycero-3-phosphocholine

DLPC or DC12:0PC=1,2-dilauroyl-sn-glycero-3-phosphocholine

DMPC or DC14:0PC=1,2-dimyristoyl-sn-glycero-3-phosphocholine

DPPC or DC16:0PC=1,2-dipalmitoyl-sn-glycero-3-phosphocholine

DSPC or DC18:0PC=1,2-distearoyl-sn-glycero-3-phosphocholine

DAPC or DC20:0PC=1,2-diarachidoyl-sn-glycero-3-phosphocholine

DBPC or DC22:0PC=1,2-dibehenoyl-sn-glycero-3-phosphocholine

DC16:1PC=1,2-dipalmitoleoyl-sn-glycero-3-phosphocholine

DC20:1PC=1,2-dieicosenoyl-sn-glycero-3-phosphocholine

DC22:1PC=1,2-dierucoyl-sn-glycero-3-phosphocholine

DPPG=1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol

DOPG =1,2-dioleoyl-sn-glycero-3-phosphoglycerol

The term "biologically active compound" as used herein means a chemicalcompound that is known in the art as having utility for modulatingbiological processes so as to achieve a desired effect in modulation ortreatment of an undesired existing condition in a living being, such asa medical, agricultural or cosmetic effect. Thus, biologically activesubstances are generally selected from the broad categories ofmedicaments, pharmaceuticals, radioisotopes, agricultural products andcosmetics.

Therapeutic biologically active compounds, or drugs for encapsulation inthe methods and compositions of this invention may be selected from thegeneral group consisting of anti-neoplastic agents, anti-infectiveagents, hormones, anti-depressives, antiviral agents, anti-nociceptiveagents, anxiolytics and biologics.

Representative examples of anti-neoplastic agents useful in thecompositions and methods of the present invention include methotrexate,taxol, tumor necrosis factor, chlorambucil, interleukins, etoposide,cytarabine, fluorouracil and vinblastine.

Representative examples of anti-infective agents useful in thecompositions and methods of the present invention include pentamidine,metronidazole, penicillin, cephalexin, tetracyclin and chloramphenicol.

Representative examples of anti-viral agents useful in the compositionand methods of the present invention include dideoxycytidine,zidovudine, acyclovir, interferons, dideoxyinosine and ganciclovir.

Representative examples of anxiolytics and sedatives useful in thecompositions and methods of the invention include benzodiazepines suchas diazepam, barbiturates such as phenobarbital and other compounds suchas buspirone and haloperidol.

Representative examples of hormones useful in the compositions andmethods of the present invention include estradiol, prednisone, insulin,growth hormone, erythropoietin, and prostaglandins.

Representative examples of anti-depressives useful in the compositionsand methods of the present invention include fluoxetine, trazodone,imipramine, and doxepin.

Representative examples of anti-nociceptives useful in the compositionsand methods of the present invention include hydromorphine, oxycodone,fentanyl, morphine and meperidine.

The term "biologics" encompasses nucleic acids (DNA and RNA), proteinsand peptides, and includes compounds such as cytokines, hormones(pituitary and hypophyseal hormones), growth factors, vaccines etc. Ofparticular interest are interleukin-2, insulin-like growth factor-1,interferons, insulin, heparin, leuprolide, granulocyte colonystimulating factor (GCSF), granulocyte-macrophage colony stimulatingfactor (GM-CSF), tumor necrosis factor, inhibin, tumor growth factoralpha and beta, Mullerian inhibitory substance, calcitonin, andhepatitis B vaccine.

The biologically active substance can be employed in the presentinvention in various forms, such as molecular complexes or biologicallyacceptable salts. Representative examples of such salts are succinate,hydrochloride, hydrobromide, sulfate, phosphate, nitrate, borate,acetate, maleate, tartrate, salicylate, metal salts (e.g., alkali oralkaline earth), ammonium or amine salts (e.g., quarternary ammonium)and the like. Furthermore, derivatives of the active substances such asesters, amides, and ethers which have desirable retention and releasecharacteristics but which are readily hydrolyzed by physiological pH orenzymes in vivo can also be involved.

The method of this invention is practiced by substituting a short chainlipid of a particular type, such as a phospholipid, generally one havingless than 12 carbons in the hydrocarbon chain, with one having 1 or moreadditional carbons in the chain, generally with one having 13 to about22 carbons. For instance, the encapsulation efficiency of a liposomalformulation can be increased by substituting a 13 carbon lipid for a 12carbon lipid, or by substituting a 16 carbon lipid for a 14 carbonlipid, wherein the chemical structure of the two lipids is otherwisesubstantially similar, and the composition of the liposomal formulationis otherwise unchanged. As shown in Table 2, the encapsulationefficiency of cytarabine increased monotonically from 0.2% up to 56.9%when a 12 carbon saturated PC was replaced by a longer chain saturatedPC of 14 to 18 carbons. As is also shown in Table 2, the encapsulationefficiency of cytarabine increased from 30.1% to 44.6% when a 16 carbonunsaturated PC was replaced by an 18 carbon unsaturated PC having thesame number of double bonds at the same position (position 9) in theacyl chain, and increased from 44.6% to 57.5% when an 18 carbonunsaturated PC was replaced by a 20 carbon unsaturated PC having thesame number of double bonds, but located at a different position(position 11) in the acyl chain. However, an increase in the percentcompound encapsulated will not necessarily result from substituting a 13carbon unsaturated lipid for a 12 carbon saturated lipid, or a 13 carbonphospholipid for a 12 carbon phospholipid, if the chemical structures ofthe two lipids differ substantially in other respects, i.e., if thelipid head groups, the stereochemistry (such as changing a cis to atrans, or a D-type to an L-type, or exhanging other types ofstereoisomers), or the number of double bonds within the two unsaturatedlipids is different.

Preferably, the method of increasing the encapsulation efficiency of aliposomal formulation is applied to techniques for encapsulatingbiologically active substances into MVL. Generally, the encapsulationefficiency of a liposomal formulation can be increased by at least 30%,and in MVL the percent encapsulation of the active substance isincreased to as great as 65%, or even to as great as 85%, depending uponthe chemical characteristics of the lipids and biologically activesubstances used in the formulation. For example, it has been discoveredthat the efficiency of encapsulating leuprolide into a multivesicularliposomal formulation can be increased five fold by increasing thenumber of carbons from 12 to 20 in a saturated phospholipid used information of the liposome, while the particle diameter is increased onlyfrom 10.9 μm to 15.1 μm. The diameter of liposomes of the invention isgenerally less than 50 μm and preferably less than 25 μm. The resultingchange in the lipocrit of the formulations is only from 35.8 to 40.1%for a similar drug concentration in the liposome suspension. Inaddition, for a given drug concentration in the final liposomesuspension, an MLV formulation would have a significantly higherlipocrit than a MVL formulation. For instance, an MLV formulation ofcytarabine using DSPC has a lipocrit of 24.6 for a drug concentration of3.5 mg/mL; whereas an MVL formulation using DSPC has a lipocrit of 26.8,and a drug concentration of 10.2 mg/mL, which is three times higher.

This finding is not particular to multivesicular liposomes. An increasein the encapsulation efficiency of multilamellar liposomes is also seenwith increase in the number of carbons in the chain of at least one ofthe lipids used in manufacture, as is illustrated in Example 8 below.However, in multilamellar liposomes the increase is not generally asgreat as in multivesicular lipcsomes, and is generally in the range from6 to 50%. For instance, as shown in Example 8, in a multilamellarformulation an increase in the number of carbons in the phospholipidcarbon chain from 14 to 18 resulted in an increase in encapsulationefficiency from 6.5 to 44.2%.

An increase in encapsulation efficiency with increasing number ofcarbons in the phospholipid used in making the liposome is also obtainedwhen an unsaturated phospholipid is introduced into the formulation.Generally, in MVL, increasing the number of carbons in the chain of theunsaturated phospholipid results in an increase of encapsulationefficiency in the range of from 30% to 80% with relatively smallcorresponding increase in particle diameter. For instance, whenleuprolide is encapsulated into multivesicular liposomes comprising anunsaturated phospholipid with one double bond, and the number of carbonsin the phospholipid carbon chain is increased from 18 to 22, theencapsulation efficiency increases from 55.4% to 83.3%, while the meanparticle diameter increases from 9.7 to 14.3 μm.

In principle, there is no upper limit on the length of the lipid carbonchain that can be used to increase the encapsulation efficiency, exceptthat imposed by the phase transition temperature of the lipids atprocess conditions. In formulating MVL used in the method of thisinvention there are three temperatures to take into consideration: theboiling point of the solvent; the gel-liquid crysalline phase transitiontemperature of the lipid; and the process temperature. Of these, theboiling point of the solvent should be the highest, and the phasetransition temperature of the lipid(s) should be the coolest, with theprocess temperature in between. Thus the chain length of the lipidshould be selected to have a gel-liquid crystalline phase transitiontemperature less than 100° C. at STP for an aqueous solvent.Additionally, for best encapsulation efficiency, when saturated lipidsare used in the formulation of multivesicular liposomes, it is usuallyrequired that the emulsification be performed at a temperature above thegel-liquid crystalline transition temperature of the lipid used inmanufacture of the solvent spherules.

As used herein, the "shelf life" of a liposomal formulation is relatedto the rate of release of the encapsulated substance from a liposomalformulation in a storage solution, for instance normal saline (0.9%sodium chloride), at a storage temperature, for instance at 4° C. Theshelf life of multivesicular liposomal formulations whose encapsulationefficiency is increased by incorporation of a long chain amphipathiclipid, for instance one containing from 13 to 28 carbons, as taughtherein, is also significantly increased in proportion to the increase inthe number of carbons in the carbon chain.

In one embodiment, therefore, the present invention provides a liposomalcomposition comprising a pharmacologically active amount of abiologically active compound encapsulated in a multivesicular liposomeformulation wherein the formulation comprises at least one amphipathiclipid having a carbon chain containing 13 to 28, and preferably from 13to 22 carbons. Such multivesicular liposomes can be made by the processdescribed herein and inherently possess the capacity to encapsulate thebiologically active compound with a greater efficiency than any otherknown type of liposome and with an efficiency that increases in directproportion with the number of carbons in the carbon chain of theamphipathic lipid(s) used in preparation of the formulation. Preferably,at least one of the long chain amphipathic lipids in the multivesicularliposomes is a phospholipid, and is most preferably a phosphocholine.

The compositions and methods of the invention present severaladvantages, especially to the pharmaceutical industry. Thus, improvedencapsulation efficiency results in improved yields and cost savings.

The methods of achieving a high encapsulation of compound of thisinvention generally allow for production of liposomal formulationscharacterized by a higher drug:lipid ratio for any given drug than canbe achieved by other methods of encapsulation. A high drug-lipid ratiois of practical importance to liposomal systems for in vivoadministration of drugs and other therapeutic compounds becauseliposomal formulations made for injection into a body must generallyhave a lipocrit less than about 60% to be considered injectable, yetpreferably contain a therapeutically effective amount of theencapsulated drug in a single dose to avoid repeated injections.Therefore, formulations that combine a high drug encapsulationefficiency and a high drug:lipid ratio are especially preferred fortherapeutic treatments in which drugs are encapsulated in liposomalformulations for administration to patients by injection and can beachieved using the methods of this invention.

The following examples illustrate the manner in which the invention canbe practiced. It is understood, however, that the examples are for thepurpose of illustration, and the invention is not to be regarded aslimited to any of the specific materials or conditions therein.

EXAMPLE 1

Preparation of Multivesicular Liposome Formulations.

The first step in the preparation of multivesicular liposomes was theformation of a `water-in-oil` emulsion. The first emulsion was preparedfrom a test lipid component containing 4 or 5 mL of a solution made of13.20 mM of a test phosphatidylcholine (PC) with a chain length rangingfrom 12 to 22 carbons (either saturated or unsaturated) (Avanti PolarLipids Inc., Alabaster, Ala.), 19.88 mM cholesterol (Spectrum ChemicalManufacturing Corporation, Gardena, Calif.), 2.79 mM1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG), and 2.44 mMtriolein (Avanti Polar Lipids) in chloroform. The lipid component wasmixed with 4 or 5 mL of an aqueous drug solution to form a first aqueoussolution containing one of the following drugs: cytarabine, leuprolideor morphine.

In the first mixing step, the mixture was emulsified using a TK model KAutohomogenizer at a speed of 9,000 rpm for 8 min. To the resultingfirst emulsion was added 20 mL of a solution containing 4 wt % glucose(Sigma Chemical Co., St. Louis, Mo.) and 40 mM lysine (Degussa Corp.,Marceau, France). In the second mixing step, the mixture was emulsifiedagain at a speed of 4,000 rpm for 60 sec using the model KAutohomogenizer. The resulting second emulsion, a`water-in-oil-in-water` double emulsion, was transferred to a 1 LErlenmeyer flask containing 30 mL of a solution of 4 weight % glucoseand 40 mM lysine, with gentle swirling. Chloroform was evaporated bypassing nitrogen gas over the emulsion at 37° C. for 20 min with gentleshaking. The resulting multivesicular liposomes were washed twice with50 mL normal saline (0.9% sodium chloride) (McGaw Inc., Irvine, Calif.),or acidified saline, if needed to remove morphine crystals in somemorphine formulations. The washed liposomes were centrifuged at 600×g ona bench top centrifuge, and the supernatant was decanted to yield a"final suspension" of 2-8 mL. In cases in which the main phasetransition temperature of the test PC was close to or higher than theroom temperature, an elevated temperature in the first mixing and secondmixing step was usually used to make the multivesicular liposomes.

EXAMPLE 2

Determination of Encapsulation of Compound, Lipocrit, Percent Free Drugand Particle Size Distribution.

Each of the above-described preparations was characterized as follows:

Supernatant was obtained by centrifuging 0.2 mL of suspension for 3 minat 600×g in an Eppendorf centrifuge tube. For cytarabine and morphineformulations, 50 μL of the supernatant was withdrawn and pipetted into aglass tube containing 2 mL of 3:1 v/v isopropyl alcohol:1N hydrochloricacid (Fisher Chemical, Fair Lawn, N.J.), followed by rigorous mixing toobtain a clear solution. The absorbance at 280 nm for cytarabine or at285 nm for morphine was measured on a U-2000 spectrophotometer (HitachiInstruments Inc., Danbury, Conn.). For leuprolide formulations, 50 μL ofthe supernatant was withdrawn and pipetted into a glass tube containing2 mL of 1:1 isopropyl alcohol:water titrated to pH 10 using 0.1 Nammonium hydroxide, followed by rigorous mixing to obtain a clearsolution. The absorbance at 280 nm was then measured on thespectrophotometer. Similar absorbance assays were carried out for theliposome suspension except that a greater dilution was needed if thesuspension contained a higher concentration of drug. A referenceabsorbance standard was established for each drug based on solutions ofthe test drug of known concentration in the test dissolving solution.The concentrations of drug in the suspension and supernatant werecalculated based upon the reference absorbance standard using knownmethods.

Percent encapsulation of compound is the ratio of the amount of drugencapsulated to the amount of compound introduced prior to encapsulationtimes 100. Percent free drug is the ratio of the drug concentrationexterior to the liposomes in the liposome suspension to the total amountof drug in the liposome suspension, multiplied by (100 minus thelipocrit).

Lipocrit was determined by the hematocrit method. About 50 μL of themultivesicular liposome suspension were taken up into a capillary tube.One end of the tube was sealed while ensuring that there were no airbubbles. Upon centrifugation at 600×g for 10 min, the suspensionseparated into a pellet layer and a supernatant layer. The percent ratioof the length of the tube occupied by the pellet to that occupied by thesuspension was determined to calculate the lipocrit.

Particle size distribution and the mean diameter were determined by themethod of laser light diffraction using an LA-500 or LA-910 ParticleSize Analyzer (Horiba Inc., Irvine, Calif.).

When measurements were carried out in triplicate, the relative standarddeviations were less than 3% for the encapsulation of compound and lessthan 10% for the total drug concentration, percent free drug, lipocrit,and mean particle diameter, as shown by the data in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Determination of the reproducibility of Cytarabine encapsulation              in multivesicular liposomes of varied chain length and saturation                     [Cytara- % Encap-               Mean Par-                             Sample  bine].sup.1                                                                            sulation of     Lipocrit                                                                             ticle Dia-                            Preparation                                                                           mg/mL    compound % Free (in %) meter, μm                          ______________________________________                                        DC16:1  10.0     30.6     4.6    30.7    9.9                                  Sample 1                                                                      DC16:1  10.6     29.6     5.5    30.4   10.0                                  Sample 2                                                                      DC16:1  9.8      30.1     5.4    29.7   10.1                                  Sample 3                                                                      Mean ± SD                                                                          10.1 ± 0.4                                                                          30.1 ± 0.5                                                                          5.2 ± 0.5                                                                         30.3 ± 0.5                                                                        10.0 ± 0.1                         RSD, % of                                                                               4.0%     1.7%     9.6%   1.6%   1.0%                                mean                                                                          DC18:1  9.4      45.3     2.5    29.1    9.9                                  Sample 1                                                                      DC18:1  9.7      44.7     2.9    29.5    9.9                                  Sample 2                                                                      DC18:1  9.8      43.7     2.7    30.2    9.8                                  Sample 3                                                                      Mean ± SD                                                                          9.6 ± 0.2                                                                           44.6 ± 0.8                                                                          2.7 ± 0.2                                                                         29.6 ± 0.6                                                                        9.9 ± 0.1                          RSD, % of                                                                               2.1%     1.8%     7.4%   2.0%   1.0%                                Mean                                                                          DC20:1  10.0     57.8     3.0    28.8   11.0                                  Sample 1                                                                      DC20:1  10.2     55.7     2.9    29.8   10.9                                  Sample 2                                                                      DC20:1  9.6      58.8     3.4    28.4   11.1                                  Sample 3                                                                      Mean ± SD                                                                          9.9 ± 0.3                                                                           57.4 ± 1.6                                                                          3.1 ± 0.3                                                                         29.0 ± 0.7                                                                        11.0 ± 0.1                         RSD, % of                                                                               3.0%     2.8%     9.7%   2.4%   0.9%                                Mean                                                                          ______________________________________                                         .sup.1 [Cytarabine] stands for the concentration of cytarabine in the         final liposome suspension.                                               

EXAMPLE 3

The Dependence of Percent Encapsulation of Compound Upon PC Chain Lengthfor Cytarabine in Multivesicular Liposomes.

Cytarabine was encapsulated into multivesicular liposomes as describedabove, but with 20 mg/ml cytarabine in 136 mM hydrochloric acid as thefirst aqueous solution. Five mL of the first aqueous solution was mixedwith 5 mL of the test lipid combination solution to produce the firstemulsion. Seven test lipid combination solutions containing differentphospholipids with chain lengths from 12 to 20 carbons in length wereprepared, each containing one of the following phospholipids: DC12:0PC,DC14:0PC, DC16:0PC, DC18:0PC, DC16:1PC, DC18:1PC, or DC20:1PC (AvantiPolar Lipids, Inc. Alabaster, Ala.). For the saturated phospholipids,DC14:0PC, DC16:0PC, DC18:0PC, the emulsification was carried out atabout 45, 55 and 60° C., respectively. For the unsaturatedphospholipids, DC16:1PC, DC18:1PC, and DC20:1PC, the emulsification wascarried out at the ambient temperature (˜23° C.). The percentencapsulation of compound for each formulation was determined, and theresults are summarized in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        Effect of phospholipid chain length on encapsulation                          efficiency of cytarabine in multivesicular liposomes                                   [Cytar- % Encapsulation   Lipocrit                                   Phospholipid                                                                           abine].sup.1                                                                          of compound % Free                                                                              (in %)                                                                              Mean                                 ______________________________________                                        Saturated PC                                                                  DC12:0PC 0.4     0.2         --    --    --                                   DC14:0PC 6.2     31.1        0.9   22.0  9.0                                  DC16:0PC 10.7    54.6        1.3   28.5  9.8                                  DC18:0PC 10.2    56.9        0.8   26.8  9.7                                  Unsaturated PC                                                                DC16:1PC 10.1    30.1        5.2   30.3  10.0                                 DC18:1PC 9.6     44.6        2.7   29.6  9.9                                  DC20:1PC 9.9     57.5        3.1   29.0  11.0                                 ______________________________________                                         .sup.1 [Cytarabine] stands for the concentration of cytarabine in the         final liposome suspension.                                               

The data in Table 2 show that the encapsulation efficiency forcytarabine increases as the length of the carbon chain in thephospholipid increases, for both the saturated and unsaturatedphospholipids. On the other hand, no obvious dependence of lipocrit orparticle size upon lipid chain length is shown.

EXAMPLE 4

Freedom of Dependence of Percent Encapsulation of Compound Upon OtherLipids.

To determine whether the trend of increase in encapsulation of compounddepends upon the chain length or number of carbons in a lipid other thanthe PC in the liposome formulation, DPPG in the cytarabine formulationsof Example 3 was replaced with DOPG, which has a different carbon chainlength and saturation. Three different lipid combination solutions wereprepared, each one containing DOPG and one of the followingphospholipids: DC16:1PC, DC18:1PC, or DC20:1PC. Since the phospholipidsare unsaturated, the emulsification was carried out at the ambienttemperature (≈23° C.). The results of these experiments are shown belowin Table 3.

                  TABLE 3                                                         ______________________________________                                        Effect of varying the chain length of the phospholipid on encapsulation       efficiency for cytarabine encapsulation into multivesicular liposomes                             % Encap-                                                             [Cytar-  sulation of                                                                            %     Lipocrit                                                                            Mean                                 Phospholipid                                                                             abine].sup.1                                                                           compound Free  (in %)                                                                              Particle                             ______________________________________                                        DOPG-DC16:1PC                                                                            7.4      36.8     2.1   25.8  9.0                                  DOPG-DC18:1PC                                                                            9.6      47.3     1.7   30.5  10.0                                 DOPG-DC20:1PC                                                                            10.7     53.2     2.3   36.6  11.2                                 ______________________________________                                         .sup.1 [Cytarabine] stands for the concentration of cytarabine in the         final liposome suspension.                                               

The results summarized in Table 3 show that even with the replacement ofDPPG by DOPG, the trend of increasing encapsulation efficiency withincreasing chain length in the liposome does not change. Therelationship holds for both saturated and unsaturated phospholipids. Aliposome formulated using DOPG and DC16:1PC, an unsaturated phospholipidwith a single double bond and a 16 carbon chain, encapsulated cytarabinewith 36.8% encapsulation efficiency; while the combination of DOPG andDC20:1PC, a phospholipid with a single double bond and a 20 carbonchain, encapsulated cytarabine with 53.2% encapsulation of compound.

EXAMPLE 5

Preparation of Multilamellar Liposomes (MLV) Encapsulating Cytarabine.

Multilamellar liposomes (MLV) were prepared by adding 20 mg/mLcytarabine (The Upjohn Co., Kalamazoo, Mich.) solution preheated to60-65° C. into a test tube containing one of a series of testphosphatidylcholine (PC) having chain lengths ranging from 14 to 18carbons (DCn:OPC, n=14-18) to make a 100 mM lipid dispersion. At 10minute intervals for a total of five times, the dispersion was stirredin the test tube for 30 seconds using a vortexer (Baxter S/P VortexMixer) at the maximum speed. The dispersion was then allowed to undergothree cooling-heating cycles across the phase transition temperature ofthe test PC to facilitate drug equilibration across the bilayermembranes of the liposomes. The MLVs were then pelleted bycentrifugation at 600×g and washed with normal saline (20:1 volumeratio). To ensure appropriate pellet washing, saline wash tests wereconducted at various washing temperatures. It was found that arelatively thorough wash can be achieved with only two saline washes ifthe washing temperature is kept below the gel-liquid crystallinetransition temperature of the test PC. For this reason, MLV made usingDC14:0PC as the test PC were washed at 4° C., and those made usingDC16:0PC or DC18:0PC as the test PC were washed at the ambienttemperature. After the wash, the pellet was resuspended in normalsaline.

EXAMPLE 6

Morphine

Morphine was encapsulated into multivesicular liposomes as described inExample 1 and characterized as described in Example 2 above, but with 36mg/ml morphine sulfate pentahydrate (Mallinckrodt Chemical Inc., St.Louis, Mo.) in 100 mM hydrochloric acid as the first aqueous solution.Five mL of the first aqueous solution was mixed with 5 mL of the lipidcombination solution to produce the first emulsion. For the saturatedphospholipids, DC14:0PC, and DC18:0PC, the emulsification was carriedout at two different temperatures, about 45 and 60° C., respectively.The results are summarized in Table 4.

                  TABLE 4                                                         ______________________________________                                        Effect of phospholipid chain length on encapsulation                          efficiency of morphine into multivesicular liposomes                                                                  Mean                                                     % Encap-             Particle                                       [Morphine].sup.1                                                                        sulation of    Lipocrit                                                                            Diameter                              Phospholipid                                                                           (mg/mL)   compound % Free                                                                              (in %)                                                                              (μm)                               ______________________________________                                        Saturated PC                                                                  DC14:0PC 13.9      38.7     1.7   27.6  9.2                                   DC18:0PC 20.1      55.6     0.7   32.9  9.5                                   Unsaturated PC                                                                DC16:1PC 12.8      35.5     7.7   20.2  9.8                                   DC18:1PC 21.5      59.8     1.3   36.5  9.5                                   DC20:1PC 24.6      68.3     0.9   37.6  9.6                                   ______________________________________                                         .sup.1 [Morphine] stands for the concentration of morphine in the final       liposome suspension                                                      

As shown by the data in Table 4, the encapsulation of compound increasesas the PC chain length increases for both saturated and unsaturatedphospholipids.

EXAMPLE 7

Leuprolide

Leuprolide was encapsulated into multivesicular liposomes andcharacterized as described in Examples 1 and 2 above, but with thefollowing modifications. Test lipid combination solutions of four mLmade of 39.60 mM phosphatidylcholine (PC) of various chain length andsaturation, 59.64 mM cholesterol, 9.37 mM DPPG, and 7.32 mM triolein inchloroform were each mixed with 4 mL of 10 mg/ml leuprolide acetate(Bachem Bioscience Inc., King of Prussia, Pa.) in 100 mM phosphoric acidto produce the first emulsion. The results are summarized in Table 5.

                  TABLE 5                                                         ______________________________________                                        Effect of phospholipid chain length on efficiency of                          encapsulating leuprolide into multivesicular liposomes                                                                Mean                                                      % Encap-            Particle                                       [Leuprolide].sup.1                                                                       sulation of                                                                            %    Lipocrit                                                                            Diameter                              Phospholipid                                                                           (mg/mL)    compound Free (in. %)                                                                             (μm)                               ______________________________________                                        Saturated PC                                                                  DC12:0PC 2.0        9.8      5.9  35.8  10.9                                  DC14:0PC 2.0        12.3     3.1  35.8  13.2                                  DC16:0PC 1.6        20.1     10.9 35.7  15.9                                  DC18:0PC 2.4        46.7     3.5  37.4  15.5                                  DC20:0PC 2.6        50.7     2.9  40.1  15.1                                  Unsaturated PC                                                                DC18:1PC 4.1        55.4     2.2  39.3  9.7                                   DC22:1PC 3.4        83.3     1.1  56.1  14.3                                  ______________________________________                                         .sup.1 [Leuprolide] stands for the concentration of leuprolide in the         final liposome suspension.                                               

As shown by the data in Table 5, the drug encapsulation efficiencyincreases as the PC chain length increases for both saturated andunsaturated phospholipids.

EXAMPLE 8

Dependence on Chain Length of Encapsulation of Cytarabine IntoMultilamellar Liposomes.

In order to determine whether the relationship between percentencapsulation of compound and lipid chain length applies to liposomesother than multivesicular liposomes, cytarabine was encapsulated intothree different multilamellar liposomes, each prepared using aphospholipid with saturated carbon chains of 14, 16 or 18 carbons:DC14:0PC, DC16:0PC or DC18:0PC. The results are summarized in Table 6below. From these results it is seen that, as is the case withmultivesicular liposomes, the drug encapsulation efficiency of themultilamellar liposomes increases with the length of the carbon chain inthe lipid.

                  TABLE 6                                                         ______________________________________                                        Effect of chain length on efficiency of encapsulating                         cytarabine in multilamellar liposomes                                                                                 Mean                                                     % Encap-             Particle                                      [Cytarabine].sup.1                                                                       sulation of                                                                            %     Lipocrit                                                                            Diameter                              Phospholipid                                                                          (mg/mL)    compound Free  (in %)                                                                              (μm)                               ______________________________________                                        DC14:0PC                                                                              1.03       6.5      1.2   14.0  4.7                                   DC16:0PC                                                                              1.85       11.6     1.1   16.7  4.1                                   DC18:0PC                                                                              3.54       44.2     0.4   24.6  4.7                                   ______________________________________                                         .sup.1 [Cytarabine] stands for the concentration of cytarabine in the         final liposome suspension                                                

EXAMPLE 9

Dependence of Shelf-life on Chain Length and Chain Saturation forMultivesicular Liposomes Containing Cytarabine.

To investigate the effects of lipid chain length on the shelf-life ofmultivesicular liposome formulations, a series of real time stabilitystudies were conducted on formulations of multivesicular liposomescontaining cytarabine and DC14:0PC, DC16:0PC and DC18:1PC. Theformulations were made as described in Example 1 and characterized asdescribed in Example 2. The results of the above stability studies aresummarized in FIGS. 1-3. In this experiment, the shelf life stability ofthe formulations containing saturated PC with a carbon chain length of14 or 16 carbons was greater than that of the formulation containing anunsaturated PC with a chain length of 18 carbons.

The foregoing description of the invention is exemplary for purposes ofillustration and explanation. It should be understood that variousmodifications can be made without departing from the spirit and scope ofthe invention. Accordingly, the following claims are intended to beinterpreted to embrace all such modifications.

We claims:
 1. A method for increasing the percent of at least onecompound encapsulated in a liposome made from a given formulationcomprising:(a) forming a multivesicular liposome, by any conventionalmeans, containing at least one encapsulated compound and having at leastone first amphipathic lipid having a carbon chain from about 1 to about12 carbons in the lipid component of the formulation; (b) determiningthe percent amount of encapsulated compound; (c) substituting said atleast one first amphipathic lipid having a carbon chain from about 1 toabout 12 carbons in the lipid component of the formulation with at leastone second amphipathic lipid with a substantially similar chemicalstructure, having from 1 to 16 more carbons in a fatty acyl chain of thesecond lipid;wherein the increased number of carbons in the fatty acylchain of the second lipid results in an increase in the percent of saidat least one compound encapsulated in the formulation.
 2. The method ofclaim 1 wherein the percent of the compound encapsulated is increased toa value between about 7.5 and 50 percent.
 3. The method of claim 1wherein the number of carbons is increased from 12 or less to a numberin the range from 13 to about
 22. 4. The method of claim 3 wherein theamphipathic lipid is a phospholipid.
 5. The method of claim 3 whereinthe amphipathic lipid is a saturated phospholipid.
 6. The method ofclaim 3 wherein the percent of the compound encapsulated is increased toa value between about 30 and 85 percent.
 7. The method of claim 4wherein the phospholipid is selected from the group consisting of1,2-dioleoyl-sn-glycero3-phosphocholine,1,2-dilauroyl-sn-glycero-3-phosphocholine,1,2-dimyristoy-sn-glycero-3-phosphocholine,1,2-dipalmitoyl-sn-glycero-3-phosphocholine,1,2-distearoyl-sn-glycero-3-phosphocholine,1,2-diarachidoyl-sn-glycero-phosphocholine,1,2-dibehenoyl-sn-glycero-3-phospbocholine,1,2-dipalmitoleoyl-sn-glycero-3-phosphocholine,1,2-dieicosenoyl-sn-glycero-3-phosphocholine,1,2-dierucoyl-sn-glycero-3-phosphocholine,1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol and1,2-dioleoyl-sn-glycero-3-phosphoglycerol,1,2-dioleoyl-sn-glycero-3-phosphoglycerol.
 8. The method of claim 1wherein the fatty acyl chain in the first amphipathic lipid contains 14carbons and the increase is by a minimum of 2 carbons.
 9. The method ofclam 1 wherein the fatty acyl chain in the first lipid contains 16carbons and the increase is by a mininimum of 2 carbons.
 10. The methodof claim 1 wherein the fatty acyl chain in the first lipid contains 18carbons and the increase is by 2 carbons.
 11. The method of claim 1 or 4wherein the increase is by 4 carbons.
 12. The method of claim 1 or 4wherein the increase is by 6 carbons or more.
 13. The method of claim 7wherein the acyl chain is an SN₁ or an SN₂ chain, or both.
 14. Themethod of claim 13 wherein the increase is in the SN₁ chain.
 15. Themethod of claim 13 wherein the increase is in the SN₂ chain.
 16. Themethod of claim 13 wherein the increase is in both the SN₁ chain and theSN₂ chain.
 17. The method of claim 1 wherein steps (a) and (b) areperformed only until the desired encapsulation efficiency is determined.18. The method of claim 1 wherein only step (c) is performed once thedesired formulation has been determined.
 19. The method of claim 1wherein 2 or more first amphipathic lipids having a carbon chain fromabout 1 to about 12 carbons in the lipid component of the formulationare substituted with second amphipathic lipids with substantiallysimilar chemical structures, but having from 1 to 16 more carbons in thefatty acyl chains of the second lipid.
 20. The method of claim 1 wherein2 or more compounds are encapsulated.